1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to a method for growing single-wall carbon nanotubes in the absence of a catalyst.
2. Discussion of the Related Art
In the field of molecular nanoelectronics, few materials show as much promise as nanotubes, and in particular carbon nanotubes, which comprise hollow cylinders of graphite, angstroms in diameter. Nanotubes can be implemented in electronic devices such as diodes and transistors, depending on the nanotube's electrical characteristics. Nanotubes are unique for their size, shape, and physical properties. Structurally a carbon-nanotube resembles a hexagonal lattice of carbon rolled into a cylinder.
Besides exhibiting intriguing quantum behaviors at low temperature, carbon nanotubes exhibit at least two important. characteristics: a nanotube can be either metallic or semiconductor depending on its chirality (i.e., conformatioral geometry). Metallic nanotubes can carry extremely large current densities with constant resistivity. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs). The two types may be covalently joined (sharing electrons). These characteristics point to nanotubes as excellent materials for making nanometer-sized semiconductor circuits.
Nanotubes can be formed as a single-wall carbon nanotube (SWNT) or a multi-wall carbon nanotube (MWNT). SWNTs can be produced by, for example, arc-discharge and laser ablation of a carbon target. Local growth of tubes on a surface can also be obtained by chemical vapor deposition (CVD). The growth of the nanotubes is made possible by the presence of metallic particles, such as Co, Fe and/or Ni, acting as catalyst. The resultant carbon nanotubes comprise contaminants, e.g., catalyst particles. For most potential nanotubes applications the use of clean nanotubes can be important, for example, where nanotubes are implemented as an active part of electric devices. The presence of contaminating atoms and particles can alter the electrical properties of the nanotubes. The metallic particles can be removed, however the process of cleaning or purifying the nanotubes can be complicated and can alter the quality of the nanotubes.
Silicon carbide (SiC) has been identified as a substrate for catalyst-free growth of multi-walled carbon nanotubes (MWNTs). SiC is a semiconductor that can crystallize under hexagonal or cubic forms. In the direction of the growth ([0001] for hexagonal SiC and [001] for cubic SiC), it is composed of alternate layers of silicon and carbon. SiC wafers of up to 4″ are commercially available. Because the layers are grouped by pairs, the two opposite faces of a wafer are different. One face is naturally composed of silicon atoms, the other face of carbon. These two faces are thus called the Si-face and the C-face.
Catalyst-free growth of MWNTs can be achieved by annealing the C-face of hexagonal silicon carbide in vacuum. However, the resulting MWNTs grow perpendicular to the surface. Based on cross-sectional transmission electron microscopy (TEM) experiments, Kunsunoki et al. (A Formation Mechanism of Carbon Nanotube films on SiC(0001), Appl. Phys. Lett. 77, 531 (200)) concluded that annealing the Si-face of SiC does not produce nanotubes but rather a graphite-like surface.
SWNTs have been identified as a potential component of electronic devices. The quality of nanotubes, e.g., their ability to act as a semiconductor, can be affected by contaminants. Therefore, a need exists for a method of catalyst-free growth of single-wall carbon nanotubes.